1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a method and apparatus of driving a liquid crystal display for improving the picture quality.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal to thereby display a picture on a display panel. An active matrix LCD includes a switching device for each liquid crystal cell of the display panel that is suitable for displaying a dynamic image. The active matrix LCD uses thin film transistors (TFT's) as switching devices.
The LCD can have a slow response time due to inherent characteristics of a liquid crystal, such as a viscosity and elasticity, as represented in the following equation (1):τr∝γd2/Δε|V2a−V2F|  (1)wherein τr is response time of a liquid crystal; d is a cell gap of liquid crystal cells; γ is a viscosity coefficient of the liquid crystal molecules; Va is a starting voltage level of a voltage across the liquid crystal; VF is a target voltage level that is a Freederick transition voltage at which liquid crystal molecules begin to make an inclined motion; and Δε is a dielectric constant anisotropy.
A twisted nematic (TN) mode liquid crystal has a response time affected by the viscosity and elasticity of the liquid crystal, as well as other physical characteristics of the liquid crystal. Typically, a TN mode liquid crystal has response times that includes a rise time of 20 to 80 ms and a fall time of 20 to 30 ms. Since these response times of the TN mode liquid crystal are longer than the interval (i.e., 16.67 ms in the case of NTSC system) of a frame in a moving picture, a voltage charge in a liquid crystal cell for a first frame progresses into a second frame prior to arriving at a target voltage for the first frame. Thus, a blurring phenomenon occurs in which the moving portion of a picture on the display panel is blurry.
FIG. 1 is a waveform diagram showing variation in brightness levels in response to changes in voltage levels of data voltage for a cell of a related art liquid crystal display. The cell of a related art LCD will not display a desired color at a desired brightness, upon implementation of a moving picture, if the display brightness BL fails to reach a target brightness level corresponding to an input of data voltage VD. This failure is due to the slow response time of the liquid crystal in changing from one voltage level of a first data voltage to another voltage level of a second data voltage, as shown in FIG. 1. Accordingly, the display quality of the LCD is reduced since the contrast ratio for the moving portions of the picture is reduced or is not commensurate with the input of data voltage VD.
To overcome the problems associated with the slow response time of liquid crystal in an LCD, a strategy of modulating a data in accordance with a change of the data, hereinafter referred to as “high-speed driving strategy” has been suggested. A high-speed driving strategy modulates input data into modulated data. The waveform diagram shown in FIG. 2 is an example of a brightness variation in response to input of a modulated data voltage in a related art high-speed driving strategy. The related art high-speed driving strategy modulates an input data voltage VD to apply a modulated data voltage MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. This high-speed driving strategy increases the difference of |Va2−VF2| in the above equation (1) such that a desired brightness level BL for the data voltage VD can be obtained in a response time that is less than an interval of a frame. In effect, the modulating the data voltage VD accelerates the speed in which the liquid crystal responds such that the brightness level BL, which is commensurate with the voltage level of the data voltage VD, is reached prior to the next frame. Thus, an LCD using a high-speed driving strategy compensates for a slow response time of the liquid crystal by modulating inputted data values such that the blurring phenomenon is reduced and the moving portion of a picture is displayed at a desired color and at a desired brightness.
Referring to FIG. 3, a related art high-speed driving strategy compares the four most significant bits MSB of a previous frame Fn−1 with those of a current frame Fn and, if there is a change of the most significant bits, selects the corresponding modulating data Mdata from a look-up table to modulate it as shown in FIG. 3. This type of high-speed driving strategy modulates only the first four most significant bits so as to reduce the size of memory needed. A high-speed driving apparatus implemented in this manner is shown in FIG. 4.
Referring to FIG. 4, a related art high-speed driving apparatus includes a frame memory 43 connected to a most significant bit bus line 42, and a look-up table 44 commonly connected to the most significant bit bus line 42 and an output terminal of the frame memory 43. The frame memory 43 stores the most significant bits MSB during one frame interval and supplies the stored data to the look-up table 44. The most significant bits MSB can be, for example, the first four 4 bits of an 8-bit RGB source data representing the voltage level of a data voltage.
The look-up table 44 compares the four most significant bits MSB of a current frame Fn inputted from the most significant bit line 42 with those of the previous frame Fn−1 inputted from the frame memory 43 to select the corresponding modulating data Mdata in accordance with a look-up table, such as Table 1 below. The modulating data Mdata is then combined with the least significant bits LSB from a less significant bit bus line 41 for application to the LCD panel.
TABLE 1Fn − 1/Fn012345001245555510123455552023455553012445554012345555123455560134555701244558123455902345510013455111245512023551301345141345151345
In the above table, the left column is the value of the data voltage determined from the MSB of the previous frame Fn−1 while an uppermost row is the value of the data voltage determined from MSB of the current frame Fn. However, the related art LCD high-speed driving strategy is designed based on the operating temperature being at a specific reference temperature, such as at a room temperature of 20° C., to determine the look-up table. Thus, if the temperature of the liquid crystal is at different operating temperature than the reference temperature that look-up table is based upon, the modulated data voltage MVD is will not be properly set and the response time of the liquid crystal may be off. As a result, the related art LCD will now have a desired brightness or moving picture that does not blur in an environment having a different temperature than the reference temperature. For example, if a temperature of the liquid crystal is more than the reference temperature, then viscosity coefficient γ of the liquid crystal molecules and a dielectric constant anisotropy Δε of the liquid crystal are increased. However, if temperature of the liquid crystal is less than the reference temperature, then a viscosity coefficient γ of the liquid crystal molecules and a dielectric constant anisotropy Δε of the liquid crystal are decreased. Because response time of the liquid crystal is inversely proportional to temperature, the related art LCD has a higher brightness than a desired brightness when operating at temperatures above the reference temperature and will have a lower brightness than a desired brightness as well as blur when operating at a temperature below the reference temperature.